feat: add MNIST-like characters dataset generator

[charlesmindee]

This PR implements a generate_character function to generate random single character images from a vocabulary to train our recognition backbones.

Linked to #255

Any feedback is welcome!

Samples:

[codecov]

Codecov Report

Merging #408 (e598a50) into main (bb611f5) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #408      +/-   ##
==========================================
- Coverage   95.83%   95.82%   -0.01%     
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  Files          91       92       +1     
  Lines        3815     3833      +18     
==========================================
+ Hits         3656     3673      +17     
- Misses        159      160       +1     

Flag	Coverage Δ
unittests	95.82% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
doctr/datasets/__init__.py	100.00% <100.00%> (ø)
doctr/datasets/character_generator.py	100.00% <100.00%> (ø)
...dels/detection/differentiable_binarization/base.py	91.35% <0.00%> (-0.62%)	⬇️

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[fg-mindee]

Thanks for the PR!
To make it more modular, could you turn your function into something deterministic please?

• one to generate an image for a given character
  (if later on, we use this for random generation, we'll just have to randomly pick within a vocab)

[fg-mindee]

Also we need some unittests, and mypy isn't happy 😅

[fg-mindee]

A few improvement suggestions in comments!

[fg-mindee]

Also quick suggestion: let's produce straight characters (we'll be able to use transforms afterwards to rotate the characters if needed)